The operation of an electromechanical brake of an elevator is such that when the brake coil is currentless, the brake remains closed as a brake pad is pressed against a braking surface by the force generated by a mechanical pressure means, e.g. a spring. When a sufficient current is conducted to the brake coil, the force produced by the magnetic field thus set up acts in a direction opposite to the force transmitted from the pressure element to the brake pad and releases the brake, permitting rotation of the traction sheave and movement of the elevator. The brake coil current needed to release the brake, the so-called operating current, is larger than the holding current, which is needed to keep the brake in the released state after it has already been released. The brake is said to be in an energized state when released, and correspondingly in a de-energized state when the brake is closed. For operating safety, it is essential to have a possibility to get the brake into the de-energized state when necessary, which can be reliably implemented by interrupting the supply of current to the brake coil.
To control the supply of electricity to electromechanical elevator brakes, contactors connected to a direct-current circuit controlling the brake are generally used. A direct voltage is obtained e.g. by means of a rectifier from an alternating-current circuit. As the contactor works on the direct-current side, it has to be relatively large. Moreover, the contactor is a mechanical element subject to wear with time. To ensure that a failure of the contactor in the direct-current circuit will not lead to a dangerous situation, the brake is additionally controlled by contactors connected to the alternating-current side, which, however, is a relatively slow process. A prior-art brake works in such manner that when the elevator stops, the control unit of the elevator drive controls a switch on the direct-current side so as to cause the brake to start braking, whereupon the control unit removes the torque from the elevator motor. After that, the contactors on the alternating-current side are opened. If the control of the direct-current side does not work or the switch has been damaged, the elevator will bound when stopping, which involves a safety risk and gives the elevator passengers a feeling of inconvenience. In addition, the control system of the elevator drive receives no feedback information regarding brake control.
In some prior-art elevator brake control circuits the contactor in the direct-current circuit is replaced by a controlled semiconductor switch, such as a transistor. A control circuit of this type for controlling an electromagnetic brake is disclosed in specification JP 2001278554. It describes a control circuit which contains a direct-current circuit comprising a brake coil, a current measuring circuit in series with it and a transistor controlling the brake coil. The direct-current circuit receives a voltage via a rectifier from an alternating-current network. In this specification, the brake is controlled by comparing the brake coil current to a reference value and controlling the transistor using the comparison value thus obtained. This arrangement is designed to reduce the noise, losses and costs of the brake system. A drawback with the brake system according to the specification in question is that the brake circuit comprises only one transistor, which means that a failure of the transistor involves a safety risk. In addition, the working condition of the transistor cannot be checked.